Protected electrode lead for use in a corrosive environment

ABSTRACT

An electrode assembly for use in an electrolytic cell having a corrosive environment comprising high temperature chlorine gas and a bath of aluminum chloride dissolved in one or more salts. The assembly comprises an electrode disposed within the cell, at least one electrical current conducting lead made of material susceptible to corrosive attack by the environment extending into the electrode, and a fluid impervious, electrically conductive sleeve disposed around the lead along at least the length thereof extending through a wall of the cell, and in contact with the electrode. The fluid impervious sleeve is effective to prevent contact of approaching corrosive constituents with the lead.

IJited States atent [1 1 Jacobs 1 1 Sept. 24, 1974 [75] Inventor:Stanley C. Jacobs, Lower Burrell,

[73] Assignee: Aluminum Company of America, Pittsburgh, Pa.

[22] Filed: Dec. 26, 1972 [21] Appl. No.: 318,236

Related US. Application Data [62] Division of Ser. No. 178,283, Sept. 7,1971, Pat. No.

[52] US. Cl 339/112, 204/243 R, 204/250, 204/279, 204/286, 204/297 R,339/116 C [51] Int. Cl. B01k 3/02, HOlr 3/02, l-I01r 13/00 3,497,4442/1970 Paiton 204/196 3,607,708 9/1971 Priscu et a1. 204/297 R 3,612,75110/1971 Adaev et a1... 204/286 X 3,630,880 12/1971 Howard r 204/2863,645,878 2/1972 Adaev et a1 204/243 R FOREIGN PATENTS OR APPLICATIONS1,103,304 12/1959 Germany 204/297 Primary Examiner-1 C. EdmundsonAttorney, Agent, or FirmElroy Strickland 5 7 ABSTRACT An electrodeassembly for use in an electrolytic cell having a corrosive environmentcomprising high temperature chlorine gas and a bath of aluminum chloridedissolved in one or more salts. The assembly comprises an electrodedisposed within the cell, at least one electrical current conductinglead made of material susceptible to corrosive attack by the environmentextending into the electrode, and a fluid impervious, electricallyconductive sleeve disposed around the lead along at least the lengththereof extending through a wall of the cell, and in contact with theelectrode. The fluid impervious sleeve is effective to prevent contactof approaching corrosive constituents with the lead. 1

4 Claims, 1 Drawing Figure PROTECTED ELECTRODE LEAD FOR USE IN ACORROSIVE ENVIRONMENT This is a division of application Ser. No.l78.283, filed Sept. 7, 1971, now US. Pat. No. 3,745,l07.

BACKGROUND OF THE INVENTION The present invention relates generally toan electrode assembly for use in corrosive atmospheres and particularlyto an electrode structure for electrolytic cells employed in theproduction-of aluminum from aluminum chloride dissolved in a molten saltbath of one or more alkali metal halides having a higher de compositionpotential than aluminum chloride.

The potential advantages of utilizing aluminum chloride as a sourcematerial in the electrolytic reduction of aluminum have long beenrecognized but the com mercial realization thereof has been impeded byan inability to solve the many problems attending the same. Among themore serious problems of long standing has been the problem of applyingand removing current from electrodes, located within electrolysis cellsand furnaces employed in the reduction process, with leads andconductors that can withstand attack by the constituents of thecorrosive environment, i.e., the alumi num chloride containing bath andchlorine gas. The electrodes in such a cell extend into the electrolyticbath, and thus are in contact therewith, for the transmission of currentthrough the bath. The electrode leads are employed to electricallyconnect the electrodes to conductors and circuits located externally ofthe cells.

The leads connected to the cell electrodes can readily be made ofelectrically conductive metal, such as copper, but such metal leads aresubject to corrosive attack by the bath and chlorine gas at theoperating temperature of the cell which results in the rapid destructionthereof. Since the melting point of aluminum is approximately 660Centigrade, electrolytic cells employed in producing aluminum fromchloride are usu ally operated at about 700 centigrade. No known metalcan withstand the corrosive attack of chlorine over approximately 550and last longer than a day or two to a few weeks depending upon themetal used and the temperatures involved.

Leads can be made of conductive non-metals, such as graphite, but whensized to carry the necessary current at a low voltage drop areimpractically large and conduct excessive amounts of heat from the cell.When such leads are reduced in size to lower heat loss and are made apractical size, they are not sufficiently conductive, i.e., theirinherent electrical resistance produces unduly large voltage drops alongthe length thereofl Thus, a solution to the problem of supplying andremoving current respectively from the anodes and cathodes ofelectrolytic cells having a corrosive environment would be the use ofconductive members extending into the electrodes that are made from ahighly conductive, low voltage drop material subject to attack by thecorrosive environment, such as a high conductivity metal, but protectedfrom the corrosive environment in an effective manner.

BRIEF SUMMARY OF THE INVENTION The present invention provides such asolution by the use of a dense, fluid impervious graphite sleeve locatedaround a current conducting lead extending into the electrode. Thesleeve preferably extends into the body of the electrode in encirclingrelation with the lead to completely enclose the lead therewith andwithin the electrolytic cell. In this manner, any of the bathconstituents or chlorine gas. tending to penetrate through the electrodeor to the metal lead extending through a wall of the cell will bestopped from reaching the conductive lead by the interposed imprevioussleeve.

The electrode assembly and structure as briefly described above, whilenot in itself rendering the electrolytic production of aluminum fromaluminum chloride a commercially viable reality, provides the solutionof a fundamental problem of long standing, which problem has effectivelyimpeded progress in this field. As such, the present inventionrepresents a marked contribution to the desired attaining of theultimate objective of providing an economically feasible and commercialmethod for reducing aluminum from aluminum chloride.

THE DRAWING The invention, along with its advantages and objectives,will best be understood from consideration of the following detaileddescription in connection with the accompanying drawing in which thesole FIGURE is a longitudinal sectional view (with portions shown inelevation) of an electrode assembly constructed in accordance with theprinciples of the present invention.

PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawing,there is illustrated an electrode assembly 10 of the invention mountedin traverse of a refractory, heat insulating wall 12 (only partiallyshown) and which may be a top or side wall defining a housing of anelectrolytic cell (not shown) having an outer metal shell 13. Such wallsfunction to confine a highly corrosive environment, for example, aswould be produced in the electrolysis of aluminum chloride dissolved ina fused salt bath, such as one or more alkali metal halides, in theproduction of metallic aluminum. The bath, and the chlorine gasgenerated in the electrolysis process, are highly penetrative and highlycorrosive to metals, particularly at the temperatures encountered insuch cells, as explained above.

The electrode assembly 10 comprises an electrode 14 having at least onemetal lead 16, in the form of an elongated bar or rod, or other suitableconfiguration, extending into an opening or bore 18 provided in the faceof the body of the electrode facing the cell wall, and a protectivesleeve 20 surrounding the metal lead along at least a portion of itslength and extending into the bore 18 to completely enclose the leadwith the electrode. The sleeve 20 is made of a material that iselectrically conductive and impervious to the fluids (i.e., the gasesand liquids) within the cell. A sleeve structure and materialparticularly suitable for the purposes of the invention is a graphitesleeve coated with pyrolytic graphite. Pyrolytic graphite is a vapordeposited form of carbon produced when a carbonaceous gas is thermallydecomposed. Carbon atoms from the gas deposite on a surface (which, inthe present case, is the external surface of a graphite sleeve) in theform of layers of carbon atoms oriented parallel to the dispositionsurface. The layers are characterized by a high degree of crystalorientation and carbon purity which form a substantially, if notcompletely, impervious coating.

In the present invention, the protective sleeve 20 is preferablyprovided with a pyrolytic graphite coating (or the sleeve may beentirely pyrolytic) to render it impervious to fluids within anelectrolytic cell. Other impervious, electrically conductive materials,however, may be employed for the protective sleeve. for example, agraphite sleeve having its pores filled and baked with coal tar pitch orwith other resins or materials which leave a carbonaceous material inthe pores of the graphite upon baking.

For electrical contact purposes, the internal crosssectional dimensionof the protective sleeve 20 and the outer cross-sectional dimension ofthe lead 16 are chosen to provide a relatively tight fit between the twoalong the length of the lead within the sleeve. In this manner goodelectrical contact and continuity exists between the lead and sleeve.

Good electrical conductivity between the electrode 14 and the sleeve 20is provided by a force fit between the two. This is accomplished bychoosing dimensions for the sleeve and the opening 18 in the body of theelectrode that provide such a fit. Ease of insertion and removal of thesleeve, with such a tit, can be accomplished by providing the outer,cross-sectional dimension of the sleeve with a slight taper lengthwisethereof, the decreasing dimension of the taper being in the direction ofthe electrode body.

The outer face of the electrode 14, as shown in the if the electrode isan anode, and removing current therefrom if the electrode is a cathode.

In the drawing, an insulating sleeve 23 is shown located around theportion of the sleeve 20 extending through the wall 12. The material ofthe sleeve 23 is preferably quartz or silicon oxynitride, though otherinsulating materials can be used. The inner diameter of the sleeve 23 issized to removably accommodate the sleeve 20, and since the material ofthe sleeve 23 is not easily prepared (i.e., machined) to provide a tightfit between the two sleeves, an elongated annular space 24 will existtherebetween as shown in the drawing, such space being somewhatexaggerated for purposes of illustration.

In an electrolytic cell having a corrosive environment of the typedescribed above, the gaseous and liquid constituents thereof in contactwith the electrode assembly tend to penetrate the electrode to theconductors feeding and removing current therefrom. In the drawing,without suitable protection for the metal lead 16, such corrosiveconstituents would tend to seep into the area of the lead through thepores of the electrode 14 and along the interface between the wall 12and the electrode 14.

In accordance with the invention, the lead 16 is thoroughly protectedfrom such an environment by the impervious sleeve 20. Such a sleevefunctions to keep any bath or gas that penetrates the electrode 14, orthat penetrates between the electrode and the cell wall 12, fromreaching the lead 16 in any substantial amounts and thereby increasesconsiderably the life of the lead. For example, with the use of thesleeve of the invention, penetration of the corrosive constituents isminimized to the extent that the lead 16 should remain in essentiallygood working condition for extended periods of time, possibly forseveral years. In this manner. the life of the lead will correspond toor more likely extend considerably beyond the operating periodsintermediate the normal shutting down of aluminum producing cells forinspection and repair purposes. Hence, any maintenance that theprotected lead of the invention might require can be attended to whenits associated cell is ordinarily shut-down for such purposes. The taperof the graphite sleeve, as described earlier. also allows the entireelectrode assembly to be removed and replaced before shut down of thecell.

In order to seal the sleeves 20 and 23 in the cell wall 12, and to sealthe sleeve 20 around the lead 16, a removable, concentric sealing glandarrangement generally designated 32 in the drawing is employed, thearrangement simultaneously insulating the electrode sleeve 20 and thelead 16 from the conductive shell 13. More particularly, the sealarrangement 32 comprises a flanged collar 36 located around the sleeve20 and attached to the shell 13 by a suitable fastening means, such asperipherally spaced bolts, extending through the portion of the collar36 adjacent the shell and threaded into the shell. The collar isinsulated from the shell and bolts by insulating bushings 37 surroundingthe bolts in the collar, by an insulating washer 38 located between thecollar and shell, and by insulating washers 38A located between thecollar and bolt heads. Between the collar 36 and the portion of thesleeve 20 adjacent thereto is disposed an insulating packing material39, such as asbestos rope, the packing material preferably extendinginto a space between the cell wall 12, 13 and the insulating sleeve 23to seal the wall about the sleeve.

Adjacent the end of the collar 36 remote from the shell 13, and disposedabout the lead 16, is located an insulating bushing 40 held in placeover the packing material 39 confined by the collar 36 by a rigid plate41 secured to the collar by fastening bolts extending through the plateand into the portionof the collar adjacent the shell. The bushing 40insulates the inner edge of the plate 41 and the collar 36 from the lead16 while simultaneously positively securing the packing 39 at the end ofthe sleeve 20 and around the metal member.

Such a concentric sealing arrangement conveniently serves to seal thecell wall 12 against leakage of the liquids and gases from the cellabout the insulating sleeve 23, and to maintain the insulation providedbetween the conductive cell shell 13 and the sleeve 20 by the sleeve 23.

The portion of the lead 16 extending through the cell wall 13 is furtherprovided with internal passages 44 for conducting a cooling fluidthrough the lead. Since the material of the lead is highly conductive,the heat within its associated electrolysis cell would be quicklyconducted to all portions of the lead including those positions in thearea of the seal arrangement 32. A cooling fluid conducted throughpassages 44 functions to maintain that portion of the lead at asubstantially uniform temperature so that expansion and contraction ofthe lead is minimized, if not effectively avoided, in the area of theseals. In this manner, the sealing function of the seals is notadversely affected by temperature changes within and without the cell.

Cooling the portion of the lead 16 remote from the interior of a cell bymeans of a circulating coolant serves an additional function incombination with the insulating sleeve 23. In an electrolysis cell, themolten electrolytic bath in contact with the electrode assembly willtend to penetrate into the area between the cell wall and the electrodetraversing the cell wall, as explained earlier. If the bath reaches theseals around the electrode adjacent the conductive shell of the cell,the bath collects and tends to create a path for current flow betweenelectrode and the conductive shell. thereby reducing the effectivenessof the electrode in the operation of the cell.

With the cooling of the lead 16 in the area of the insulating sleeve 23,any solidifiable bath material penetrating into the space 24 between theelectrode sleeve 20 and the insulating sleeve 23 will be frozen beforeit reaches the locality of the cell shell 13 and the seals 32, thesolidified material minimizing further penetration of the bath, and thesleeve 23 thereby maintaining the insulation of the electrode sleeve andlead from the conductive shell 13.

From the foregoing description it should now be ap parent that a new anduseful electrode structure and assembly has been disclosed in which animpervious, electrically conductive sleeve is employed to protect ametal lead in a highly corrosive environment, the metal providing thelow voltage drop obtainable with metal connections, and the electricalconductivity of the sleeve insuring good electrical contact between thelead and the electrode.

As explained earlier, the electrode assembly of the invention issignificant in the process of electrolytically reducing aluminum fromaluminum chloride, and though the invention does not in itself make sucha reduction commercially viable reality, it does provide a solution to along standing problem that has heretofore impeded progress in this area,and thus contributes to making the reduction process a commercialreality. The invention, however, has appreciable utility in cells andstructures other than those employed in reducing aluminum from aluminumchloride, or for example, in the electrical connection of graphiteresistance heaters located within a furnace chamber in the production ofaluminum chloride from chlorine gas, alumina bearing material andcarbon.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit and scope of the invention.

Having thus described my invention and certain embodiments thereof, 1claim:

1. A lead assembly for conducting electrical current to or from achamber having a chamber defining wall and containing a highly corrosiveenvironment, the assembly comprising at least one electricallyconductive lead member traversing said wall and extending into saidchamber, the material of said lead member being susceptible to corrosiveattack by the environment if exposed thereto,

a fluid impervious, electrically conductive sleeve disposed around atleast the portion of said lead member traversing said wall and extendinginto said chamber, the material of said sleeve being resistant to attackby the highly corrosive environment and effective to prevent approachingconstituents of said highly corrosive environment from contacting saidlead member.

2. The structure of claim 1 in which the material of the impervioussleeve is graphite, said material being coated with a pyrolytic graphitematerial.

3. The structure of claim 1 including an electrically insulating sleevedisposed around at least the portion of the impervious sleeve thattraverses the defining wall of the chamber.

4. The structure of claim 1 including means for cooling the conductivelead member along at least the portion thereof traversing the definingwall of the chamber.

2. The structure of claim 1 in which the material of the impervioussleeve is graphite, said material being coated with a pyrolytic graphitematerial.
 3. The structure of claim 1 including an electricallyinsulating sleeve disposed around at least the portion of the impervioussleeve that traverses the defining wall of the chamber.
 4. The structureof claim 1 including means for cooling the conductive lead member alongat least the portion thereof traversing the defining wall of thechamber.